Many different processes have been proposed for the combustion of fine grain dilute (low organic matter content) solid fuels. Most of these involve the direct combustion of the fuel. Amongst such fuels there are low grade oil shales which have a content of only 7 to 20 percent organic matter and which contain about 40 to 70 weight-% carbonates. When such oil shales are conbusted at above about 700.degree. C. an endothermic decomposition of carbonate takes place, such as EQU CaCO.sub.3 .revreaction.CaO+CO.sub.2
with a heat of reaction of 420 Kcal/kg CaCO.sub.3 and 965 kcal/kg carbon dioxide. Combustion of such oil shales above 800.degree. C. reduces by about 20 to 40% their already low heat value, from about 800-1200 kcal/kg to about 600-900 kcal/kg.
Carbonate decomposition results in a brittle ash, and mechanical handling and abrasion cause it to collapse with the formation of dust which travels to the boiler, leading to erosion of boiler pipes and to the stack, resulting in air pollution.
In the process proposed by Rohrbach, R., "Production of Cement and Utilization of Oil Shale as a Fuel for Energy", in "United Nations Symposium on the Development ad Utilization of Oil Shale Resources, Au. 26 to Sept. 4, 1968, Tallim, USSR," Petuchov, E. F., Editor, pages 441-446, Walgus Publishing Co. (1970), fine-grain oil shale is fed to a single fluid-bed. Primary air is fed to the bottom of the fluidized combustion chamber, and secondary air is fed to the upper layers of the fluidized bed. Some of the boiler tubes are located directly in the fluidized bed and some in the steam boiler to which the hot combustion gases and fine dust flow. In this single fluidized bed, pyrolysis, gasification, oxidation of volatile gases and of carbon, and carbonate decomposition all take place simultaneously at the bed temperature of 800.degree.-1000.degree. C. This process of direct combustion of oil shale has the following defects. A considerable portion of the heat generated is used for carbonate decomposition and to heat the ash to a high temperature, lowering the effectiveness of electric power generation and lowering the combustion temperature. Carbonate decomposition makes the ash more brittle, and it crumbles to form dust. This dust travels to the boiler where it leads to boiler tube erosion and it goes up the stack leading to air pollution. At the combustion temperature of 800.degree.-1000.degree. C., most of the sulfur reacts with the mineral ash forming sulfates which render the ash useless for further use such as Portland cement manufacture, and creating a disposal nuisance.
Other fluidized bed processes, and among these the process of Roetheli, U.S. Pat. Nos. 2,579,397 and 2,579,398 produce alternative fuels, liquid and gaseous, which are transported to another location prior to combustion and which can be used as feed materials in chemical plants. The basic concept of these processes is a different one from that of the present invention.